Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
Because the disk is rotated at a constant angular velocity, the user data rate is typically increased toward the outer diameter tracks (where the surface of the disk is spinning faster) in order to achieve a more constant linear bit density across the radius of the disk. To simplify design considerations, the data tracks are typically banded together into a number of physical zones, wherein the user data rate is constant across a zone, and increased from the inner diameter zones to the outer diameter zones. This is illustrated in FIG. 1, which shows a prior art disk format 2 comprising a number of data tracks 4, wherein the data tracks are banded together in this example to form three physical zones from the inner diameter of the disk (ZONE 1) to the outer diameter of the disk (ZONE 3).
The prior art disk format of FIG. 1 also comprises a number of servo sectors 60-6N recorded around the circumference of each data track. Each servo sector 6, may comprise a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a track address, used to position the head over a target data track during a seek operation. Each servo sector 6, may further comprise groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
The servo sectors 60-6N are typically recorded at a constant data rate from the inner to outer diameter of the disk 2, such that the servo sectors 60-6N are said to form servo wedges across the radius of the disk 2 as shown in FIG. 1. However, recording the servo data at a constant data rate across the disk radius may be undesirable because it reduces the format efficiency, particularly at the outer diameter data tracks. In addition, the data rate may affect the ability to read the servo data reliably, depending on the radial location of the head and the corresponding velocity of the servo track. In other words, the servo data may be read more reliably if the servo data rate is optimized relative to the radial location of the servo track. However, varying the servo data rate across the disk radius complicates the servo system, and in particular, the timing recovery system which attempts to synchronize to the servo sectors as well as maintain a servo timing window for activating a servo gate as the head approaches a servo sector.
There is, therefore, a need for a disk drive comprising a timing recovery system that enables the servo data rate to vary across the radius of the disk.